How can I block messages from non friends on facebook? Or maybe be invisible for non friends?

[Flopper]

Go into privacy settings and there'll be a section called 'Who can contact me' and change it to 'Strict filtering.'

I think people you have mutual friends with will still be able to send you messages but they'll go into your 'Other' box rather than straight into your inbox. It's obviously not ideal, but if you can resist the temptation of going into your other box then you should be able message your friends without being directly confronted with the hateful things people have to say. This is the only thing i can think of, sorry.

You should consider reporting the bullying to somebody.